The nuclear-coded 22 kd heat-shock protein (HSP-22) which is transported into the chloroplast and localized in the thylakoids was further characterized and found to be located in the grana lamellae (stacked thylakoids) as an extrinsic protein in the green alga Chlamydomonas reinhardtii. Inhibition of photosynthetic electron flow during heat-shock of Chlamydomonas cells was light-dependent, occurring at low-light intensities (<100 W/m2) as compared with photoinhibition at 25°C (>1000 W/m2). The site of the damage was localized at the photosystem II (PS II) reaction center. The damage was drastically increased when heat-shock treatment was carried out in the presence of the 80S ribosomal translation inhibitor, cycloheximide (CHI). Pre-incubation of Chlamydomonas cells at 42°C resulted in partial protection against photoinhibition during heat-shock, as compared with cells pre-incubated at 42°C in the presence of CHI which, therefore, did not translate the heat-shock proteins. Analysis of the thylakoid polypeptides' pattern by SDS-PAGE revealed that during heat-shock in the light, thylakoid proteins became aggregated proportionally to the light intensity. Heat-shock in the presence of CHI enhanced the aggregation process which, at low light intensities, was specific to the PS II reaction center D1-protein. The results suggest that the chloroplasts HSPs prevent damage to the PS II reaction center during heat-shock in the light.